Background: Vascular endothelial dysfunction has been recognized as an essential feature of obstructive sleep apnea (OSA). This study was designed to examine the hypothesis that OSA may impair the coronary microcirculation in patients with ST-segment elevation myocardial infarction (STEMI). Methods and Results: The present study included 100 patients with a first STEMI who underwent primary percutaneous coronary intervention (PCI) within 12h from onset. Coronary flow velocity at baseline and at maximum hyperemia was measured using a Doppler guidewire following PCI. Total ST-segment elevation was calculated at baseline and 30min after PCI. All patients underwent polysomnography at 14 days to diagnose OSA. Coronary flow velocity reserve (CFVR) was used for quantitative analysis of myocardial tissue perfusion. Systolic retrograde flow (SRF) and ST-segment resolution (STR) <50% were used as an index of microvascular injury. Forty-eight patients presented with OSA. CFVR was comparable between the 2 groups. The incidence of SRF was higher in OSA patients than in the control patients (6% vs. 31%, P=0.005). Patients with OSA had a higher incidence of STR <50% (31% vs. 60%, P=0.003). Multiple logistic regression showed that OSA was an independent positive predictor of SRF and STR <50% (odds ratio=4.46, P=0.044; odds ratio=3.79, P=0.010). Conclusions: OSA may impair myocardial tissue perfusion following primary PCI. (Circ J 2011; 75: 890-896)